This invention relates to a high frequency heating apparatus such as microwave ovens including a magnetron supplied with a variable ac output from an inverter circuit to drive the same.
A high frequency output from a magnetron is varied with variation of an input voltage or commercial power supply voltage in conventional high frequency heating apparatus, which variation of the high frequency output ill affects the cooking.
To overcome the above-described disadvantage, the inventors have considered an arrangement that a magnetron anode current is detected and an "on" period of a switching element of an inverter circuit is controlled so that the magnetron anode current is rendered constant, thereby maintaining the magnetron high frequency output at a predetermined value.
On the other hand, it takes some period of time for the magnetron to cool down by way of heat dissipation after completion of the cooking. Consequently, when another cooking is initiated without a sufficient period of time after the previous cooking, an initial temperature of the magnetron at the time of initiation of the cooking differs from case to case in accordance with a lapse of time after completion of the previous cooking. Thus, when the magnetron initial temperature at the time of the cooking initiation differs, a mode of the subsequent raise in the magnetron temperature differs from case to case. A magnet (usually, a ferrite magnet) of the magnetron is demagnetized as the temperature of the magnetron anode is raised and consequently, the strength of a magnetic field between the anode and cathode is reduced, resulting in drop of the anode voltage. Consequently, the input power and accordingly, the high frequency output are increased and decreased depending upon the anode temperature or anode voltage when the anode current is controlled to be constant as described above. Accordingly, the gross calorific value to the food to be cooked differs between the case where the magnetron temperature is low and the case where the magnetron temperature is high even when the cooking period of time is the same, resulting in variations in the degree of finishing of the cooked food. Moreover, since the magnetron temperature is raised with lapse of time after the cooking initiation, the anode voltage drops. Consequently, the high frequency output is gradually reduced during the cooking, which prevents the heating power from being constant means for resetting the counter to an initial value thereof when a first high frequency heating operation is initiated after the power supply is put to work, and subsequently, causing the counter to start the counting operation.